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I have read this pdf file about EMI filters. I'm about to construct a variable bench power supply of 12V/5A.

LM338 power supply

Will there be any usefulness to add an EMI filter to the primary windings of the toroidal transformer? As I see it, as the transformer lowers voltage so does any EMI interference.

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  • \$\begingroup\$ You could probably get away with one on the output instead. \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 26 '15 at 15:20
  • \$\begingroup\$ What is variable here? EMI protection for surges on input side will always be there in AC to DC converters \$\endgroup\$ – Umar Jun 26 '15 at 15:39
  • \$\begingroup\$ 30V at 5A is 150 watts. Most switch mode power supplies, to meet legislation, would use power factor correction at the input because of the bridge current surges. You can't do this with this type of supply topology so, I ask, is your question related to meeting legislation or based on fear about something else? Given also that your load current might be significantly rich in HF to still show itself on the AC side, you need to be clearer about what you are attempting to achieve. \$\endgroup\$ – Andy aka Jun 26 '15 at 16:28
  • \$\begingroup\$ @Andyaka: I would like to be sure that no EMI interference is present to the output. \$\endgroup\$ – machineaddict Jun 26 '15 at 18:01
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It's a linear supply, so no EMI is created by the regulator.

Unrelated to EMI, but for surges-your regulator is only rated for 40V, that's very close to the line with a light load, depending on transformer regulation etc.

Consider putting a few hundred ohms in series with 10n or so across each of the four diodes in the bridge rectifier. Or use a soft-recovery bridge. This could be important for low-level or audio work because the diode recovery causes ringing that sounds like a 100Hz or 120Hz buzz.

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